Optical communication systems are becoming more and more widespread due mainly to the very large bandwidths they offer for carrying information. The growth and diversity of lightwave networks, such as Wavelength Division Multiplexed (WDM) and Dense WDM (DWDM) networks are placing new demands on all aspects of optical networks including, for example, capacity management and provisioning, maintenance, and reliable and robust operation.
Currently, high capacity optical networks are constructed as rings and use WDM technology to achieve high bandwidth capacities. For example, WDM ring networks are commonly used in metropolitan area network (MAN) applications but can also be used in LANs and WANs.
Wavelength division multiplexed (WDM) optical networks are particularly desirable because of their restoration capabilities and suitability for minimizing the number of optical fibers for the interconnection of system nodes. A typical WDM optical ring network includes network elements with optical add/drop multiplexers (OADMs), whereby some optical channels are dropped, some are added and/or other channels are expressed or passed through.
In a ring topology each ring node is connected to exactly two other ring nodes. The OADMs are used to construct a ring network whereby adjacent OADMs are connected pair wise while the network nodes are situated so as to form a ring. In a ring network, any node can be reached from any other node using two physically separate paths, i.e. one traveling clockwise and one counter clockwise. This is used for providing protection against route failures. The use of at least two parallel fibers with traffic flowing in opposite directions provides restoration capabilities in the event of a fiber cut break.
An Optical Add/Drop Multiplexer (OADM) functions to filter or drop one or more wavelengths transiting on the ring. The optical technologies usable for producing an OADM can be placed in two main categories, namely: (1) those using fixed filtering, whereby an OADM is produced for dropping and adding a fixed wavelength, and (2) those using tunable filtering, whereby an external control determines the wavelength of the dropped and added channel.
Normally, only a single wavelength of light is used to carry optical signals from one node to another. To increase the communications bandwidth of the network, however, it is common to transmit light signals having multiple wavelengths. Additional signal channels can be added using well-known DWDM techniques wherein each channel corresponds to a different wavelength of light.
As is common practice in DWDM optical networks, OADMs are used to drop, add or express one or more optical channels. The OADM comprises a drop module adapted to generate a drop channel from the multi-wavelength input signal and an add module adapted to add a channel to the multi-wavelength output signal.
A block diagram of a prior art network hub providing 1:1 protection against equipment and link failures is shown in FIG. 1. The optical network, generally referenced 10, illustrates a popular topology of a logical star over a physical ring. The network comprises a hub 12 and a plurality of access nodes 14 connected by optical fiber links 26 to form an optical ring network. Each access node comprises an Optical Add/Drop Multiplexer (OADM) 28 connected to a plurality of line cards or transceivers 30. The plurality of line cards 30 are connected to an electrical switch 29. Each channel or wavelength is terminated at a different access node, thus establishing a dedicated point to point connection between the hub and each of the access nodes. Communications between the hub and any one access node occurs using a different wavelength. Thus, at any one time, communications at a particular wavelength occurs between the hub and a single access node. In addition, it is possible for a wavelength to be shared by one or more access nodes on the ring.
In this example network, only a single ring is shown such that communications proceeds in the clockwise direction only. A second ring can be implemented that carries communications between nodes in the opposite or counter-clockwise direction.
The majority of protection schemes currently employed in communication systems are based either on 1:1 or 1+1 protection schemes. In order to provide protection against line failures, the same information is transmitted simultaneously in both directions around the ring. This requires doubling the equipment at every access node and at the hub. Doubling the equipment enables protection against equipment failure as well as protection against line failures by transmitting data in both directions of the ring. In this example, the hub comprises double equipment and, in particular, comprises two multiplexers 16, 24, two sets of line cards 18, 22 and a single switch 20. Note that in a dual hub system, four sets of line cards are required in order to achieve the same level of protection. Note also that under normal conditions, i.e. no line failure, both paths to an access node may be used to double the available bandwidth and thus improve efficiency. Further, a second switch may be used for redundancy purposes.
Although prior art 1:1 or 1+1 protection schemes provide line and equipment protection, a major disadvantage is that they are very expensive due to the requirement of doubling the equipment within the hub and access nodes.
There is thus a need for a protection scheme for a network hub that is not based on a 1:1 or 1+1 protection and that does not require the doubling of all the equipment within the hub. In addition, the protection scheme should protect against not only equipment failures within the network hub but also against communication link failures on the optical ring.